1. Field of the Invention
The present invention relates to a sheet supplying apparatus and an image forming apparatus, and more particularly, it relates to a sheet supplying apparatus particularly used as an automatic document (original) feeder for an image forming apparatus such as a copying machine, and an image forming apparatus having such a sheet supplying apparatus.
2. Related Background Art
In a conventional sheet supplying apparatus having a movable sheet (original) stacking plate, as disclosed in U.S. Pat. No. 5,132,741 (granted on Sep. 18, 1992), sheets stacked on the stacking plate are separated one by one, and the separated sheet is sent onto a platen of an image forming apparatus to read an image on the sheet, and, after the image was read, the sheet is discharged from the platen and is re-stacked on the sheet stacking plate. Further, in such a sheet supplying apparatus, switch-back path and a closed loop path are selectively switched in accordance with a size of the sheet so that, when the closed loop path is used, after the sheet stacking plate is moved to a lower position, the above-mentioned fundamental operations are effected.
In the above-mentioned conventional sheet supplying apparatus, when the sheet are stacked and the supplying operation is started, as shown in FIG. 15, normally, in order to separate and supply a stack of sheets P set on the sheet stacking plate 101, a sheet supply roller 102 is firstly rotated in a direction shown by the arrow, and at the same time, a weight 106 for urging the sheet stack is driven in a direction shown by the arrow, with the result that the sheet(s) is sent to a nip between a separation and convey roller 103 and a separation belt 104 for preventing double-feed of sheets. The sheet separated by the separation and convey roller 103 and the separation belt 104 is conveyed downstreamly in a sheet conveying direction. However, in this conventional sheet supplying apparatus, if a large number (for example, 100 or more) of sheets are stacked on the sheet stacking plate 101, since a contacting force between the sheets is often increased due to weights of the sheets themselves, when the sheet supply roller 102 is driven, the sheet stack is sent to the separating station at a time, with the result that the sheet stack is pinched and locked between a separation regulating plate 105 for defining a path in the separating station and the sheet stacking plate 101, thereby causing the poor sheet supply. If such poor sheet supply occurs, in the sheet supplying apparatus, since the sheet does not pass by a separation sensor 107 within a predetermined time period, the sheet supplying and separation operations must be stopped in consequence of delay of separation.